1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer or the like, and more particularly it relates to an image forming apparatus in which productivity is improved when images are recorded on recording sheets by several times.
2. Related Background Art
Productivity (index for indicating the number of recordable copies per unit time) which is one of performances of an image forming apparatus such as a copying machine is generally determined by a process speed and a conveying distance (interval) between recording sheets.
The process speed is a conveying speed for the recording sheet in an image forming part. The conveying interval of the recording sheet is a distance between a trailing end of a preceding recording sheet and a leading end of a succeeding sheet when images are formed continuously. That is to say, the productivity of the copying machine can be improved by increasing the process speed or by decreasing the conveying interval.
However, the process speed greatly depends upon a processing ability of the image forming part. Accordingly, in order to increase the process speed, an image forming apparatus having a high speed processing ability must be adopted or developed, which leads in increase in the manufacturing cost and which is difficult to realize quickly.
In order to decrease the conveying interval of the recording sheet, there is a danger of arising the following problems.
In case of an image forming apparatus of electrophotographic type, an image is fixed to a recording sheet by heating toner transferred to the recording sheet by means of a fixing device. If the conveying interval is decreased, since the number of recording sheets fixed per unit time is increased, a greater amount of heat is absorbed from the fixing device. As a result, the recording sheet cannot be heated sufficiently, thereby causing poor toner fixing.
On the other hand, in case of an image forming apparatus having a first page processing function, a rear surface discharge mode in which recording sheets on which the images were formed are turned up or reversed and then are stacked on a discharge tray is used. In this case, when a switch-back system in which the recording sheet is temporarily into a reverse path is used as a recording sheet reversing system, since the succeeding recording sheet cannot be conveyed until the preceding recording sheet is discharged from the reverse path, a predetermined conveying interval is required.
Further, in a case where a retard system in which sheets are supplied one by one by means of a supply roller and friction of a separation roller is used, in an area where the sheet is conveyed only by the supply roller and the separation roller, a conveying speed of the recording sheet becomes unstable. Thus, the conveying interval must be increased accordingly.